The present invention generally relates to a keypad monitor and more particularly to a keypad monitor which is activated responsive to keypad activity. The present invention still more particularly relates to such a keypad monitor wherein the keypad monitor and associated circuitry is activated in response to received clock signals from an external clock source and deactivated in the absence of clock signals. The keypad monitor of the present invention as disclosed herein may be used to advantage in a hands-free, multiple feature telephone unit adapted for use in an Integrated Services Digital Network (ISDN).
Keypads and monitors associated therewith are well known in the art. Such keypads generally include an array of switches with each switch coupled between a respective different pair of first and second conductors. When a keypad switch is depressed or closed, a condition can then be sensed across the first and second conductors to which a particular switch is coupled to identify which switch is being depressed. Such condition sensing is performed by the keypad monitor.
Keypad monitors for identifying a particular keypad switch being closed have been implemented in integrated circuit form. The keypad monitors of such integrated circuits generally include a first plurality of inputs arranged to be coupled to the first keypad conductors and a second plurality of inputs arranged to be coupled to the second keypad conductors. The keypad monitors further generally include internal circuitry which scan the first and second plural inputs for detecting the closure of a keypad switch.
One dominant application for keypads and keypad monitors is in telephone equipment. Keypads are used in such applications for dialing telephone numbers or selecting various features provided by a particular piece of telephone equipment. Such features may include call transfer, call conference, hold, or multiple-line selection, for example.
The scanning circuitry of keypad monitors is generally responsive to clock signals applied to the keypad monitor integrated circuit from a clock source external to the keypad monitor integrated circuit. The keypad monitor integrated circuit may also include other circuitry which require the clock signals for operation
In the telephone industry, there is a strict requirement that telephone equipment which is not in use, and thus is in an idle state, consume a limited amount of power. Hence, such equipment must include a means for deactivating the equipment when not in use and activating the equipment when it is placed into service. In limited feature telephone equipment, this is generally accomplished by activating the telephone circuitry responsive to the telephone switch being placed off-hook by a user. In such a case, when the telephone is placed off-hook, activating power together with the externally derived clock signals are applied to the telephone equipment circuitry including the keypad and keypad monitor.
While the foregoing is generally successful in limited feature telephone equipment, it is not always suitable in multi-featured telephone equipment and especially in such equipment which is under the control of a microprocessor. This obtains because a user may select various telephone features in advance to taking the telephone equipment off-hook. In such a case, the keypad monitor, the microprocessor, and any other interface circuitry associated with the keypad monitor and feature selection would not be activated during such feature preselection prior to taking the telephone equipment off-hook.